1. Field of the Invention
The present invention relates to a zoom lens and an image scanner using it and, more particularly, to those permitting zooming in a state in which the object-image distance is maintained at a constant finite distance.
2. Related Background Art
Many conventional image scanners use an imaging lens constructed of a unifocal lens. The image scanners of this type are adapted to digital reading by a line sensor (CCD), so that enlargement of an image, etc., can be performed by electronic zooming. In general the electronic zooming has a problem that degradation of the image is unavoidable, however, because an image comprised of the fixed number of pixels is enlarged.
In recent years, development is quickly proceeding to achieve higher image quality for displays and printers of personal computers. With this trend image scanners are also becoming ready for higher resolution. In the future, optical zooming will become necessary for achieving a substantially higher quality of an enlarged image.
A zoom lens for an image scanner using this optical zooming is suggested, for example, in Japanese Patent Application Laid-Open No. 9-113804. In this application the zoom lens has two lens units, a first lens unit having a negative refracting power and a second unit having a positive refracting power in the stated order from the object side, and zooming is effected by varying the distance between the first unit and the second unit. The scanner in this application is, however, inferior in brightness of the edge of image field and in distortion to the conventional image scanners using the unifocal lens.
A zoom lens that makes use of a diffractive optical element in order to control the occurrence and variation of chromatic aberration of the zoom lens to a low level is suggested, for example, in U.S. Pat. No. 5,268,790. In the patent, the diffractive optical element is used in the second unit, which is a main zooming unit, and in the third unit, which is a correcting unit.
A zoom lens equipped with the diffractive optical element for correction of chromatic aberration is suggested, for example, in Japanese Patent Application Laid-Open No. 9-197274. The lens of this application has two lens units, a first unit having a positive refracting power and a second unit having a negative refracting power in the stated order from the object side, the diffractive optical element is placed in the first unit or in the second unit, and zooming from the wide-angle extreme to the telephoto extreme is effected by narrowing the spacing between the first unit and the second unit. The zoom lens suggested in this application is, however, used for lens shutter cameras, and, therefore, the optical performance thereof was insufficient as to chromatic aberration, image surface characteristics, and distortion for application to image scanners.
In general, the image scanners need to read the image faithfully throughout the entire region of an original surface and thus are required to have a resolving power of a certain fixed level or higher over the entire region of the field without distortion. In the color reading case, the requirements are, for example, that focus positions are aligned with each other among beams of respective colors of R (red), G (green), and B (blue) and that no chromatic deviation occurs in the image field. Since an image sensor used as a reading element in the image scanner has a narrow latitude, it is necessary to assure as much brightness of the edge of the image field with respect to the center as possible.
Further, the zoom lenses used in the image scanners have to have so high an optical performance as not to allow the aberration variation amount that is practically allowed for the zoom lenses, for example, including photographic lenses, video lenses, and so on.
An object of the present invention is to provide a zoom lens and an image scanner therewith that have a high optical performance throughout the entire zooming range while ensuring a zoom ratio as high as about the zoom ratio 2.0 and that are ready for color reading, by properly setting a lens configuration of each lens unit in a three-unit zoom lens and providing the second unit with a diffractive optical element.
A zoom lens of the present invention is a zoom lens comprising three lens units, a first unit having a negative refracting power, a second unit having a positive refracting power, and a third unit having a positive refracting power in the stated order from the side of an original surface, in which zooming is effected by changing an air space between the first unit and the second unit and an air space between the second unit and the third unit, wherein the second unit comprises a diffractive optical element.
Particularly, another zoom lens of the present invention is:a zoom lens comprising three lens units, a first unit having a negative refracting power, a second unit having a positive refracting power, and a third unit having a positive refracting power in the stated order from the side of an original surface, in which zooming is effected by changing an air space between the first unit and the second unit and an air space between the second unit and the third unit,
wherein the first unit has a first diffractive optical element and the second unit has a second diffractive optical element.
More specifically, the zoom lens of the present invention is characterized by either of the following features:
the first diffractive optical element corrects a variation in lateral chromatic aberration due to the zooming and the second diffractive optical element corrects variation in axial chromatic aberration due to the zooming;
a stop is placed in the second unit and the second diffractive optical element is placed near the stop;
the first diffractive optical element is attached to a negative refracting surface of a lens forming the first unit;
the following conditions are satisfied:
0.7 less than |xcex22wxc3x97xcex23w| less than 1.1
xe2x80x83and
xcex22wxc3x973w less than 0
where xcex22w and xcex23w are image magnifications at the shortest focal length of the second unit and the third unit, respectively;
where phase functions of the first and second diffractive optical elements are defined by the following equation:
xe2x80x83xcfx86i(h)=(2xcfx80/xcex)xcexa3Ci(j)hi
(where xcex is a reference wavelength, h is a height from the optic axis, i is a degree, and j is a number of each diffractive optical element),
and where al focal length at the shortest focal length of the overall system is fw,
the following conditions are satisfied:
0.0005 less than C2(1)xc3x97fw less than 0.005
0.005 less than |C2(2)xc3x97fw| less than 0.03
(C2(2) less than 0);
each of the first and second diffractive optical elements is comprised of a stack type diffraction grating in which a plurality of diffraction gratings are stacked on a glass substrate;
zooming from the shortest focal length extreme to the longest focal length extreme is effected by moving the first unit along a convex locus on the image surface side and monotonically moving the second and third units toward the original surface, and an object-image distance is constant;
in order from the original surface side, the first unit has three lenses, a (1-1) negative lens, a (1-2) negative lens, and a (1-1) positive lens, the second unit has five lenses, a (2-1) positive lens, a (2-2) positive lens, a (2-1) negative lens, a (2-2) negative lens-, and a (2-3) positive lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens;
in order from the original surface side, the first unit has four lenses, a (1-1) negative lens, a (1-1) positive lens, a (1-2) negative lens, and a (1-2) positive lens, the second unit has four lenses, a (2-1) positive lens, a (2-2) positive lens, a (2-1) negative lens, and a (2-3) positive lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens;
in order from the original surface side, the first unit has four lenses, a (1-1) negative lens, a (1-1) positive lens, a (1-2) negative lens, and a (1-2) positive lens, the second unit has five lenses, a (2-1) positive lens, a (2-2) positive lens, a (2-1) negative lens, a (2-2) negative lens, and a (2-3) positive lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens;
in order from the original surface side, the first unit has three lenses, a (1-1) negative lens, a (1-2) negative lens, and a (1-1) positive lens, the second unit has four lenses, a (2-1) positive lens, a (2-2) positive lens, a (2-1) negative lens, and a (2-2) negative lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens;
in order from the original surface side, the first unit has three lenses, a (1-1) negative lens, a (1-2) negative lens, and a (1-1) positive lens, the second unit has three lenses, a (2-1) positive lens, a (2-2) positive lens, and a (2-1) negative lens, and the third unit has three lenses, a (3-1) negative lens, a (3-1) positive lens, and a (3-2) negative lens; and so on. By xe2x80x9c(i-j) positive (negative) lensxe2x80x9d is here meant the j-th positive (negative) lens in the i-th group.
Particularly, another zoom lens of the present invention is a zoom lens comprising three lens units, a first unit having a negative refracting power, a second unit having a positive refracting power, and a third unit having a positive refracting power in the stated order from the side of an original surface, in which zooming is effected by changing an air space between the first unit and the second unit and an air space between the, second unit and the third unit,
wherein the second unit has a diffractive optical element and at least one lens unit out of the three lens units of the first, second, and third units has an aspherical surface.
Particularly, the zoom lens of the present invention is characterized by either of the following features:
the diffractive optical element corrects a variation in axial chromatic aberration due to the zooming;
a stop is placed in the second unit and the diffractive optical element is placed near the stop;
the first unit has an aspherical surface;
the second unit has an aspherical surface;
the third unit has an aspherical surface;
each of the first unit and the third unit has an aspherical surface;
where image magnifications at the shortest focal length of the second and third units are xcex22w and xcex23w, respectively, the following conditions are satisfied:
0.7 less than |xcex22wxc3x97xcex23w| less than 1.1
xe2x80x83and
xcex22wxc3x97xcex23w less than 0;
where a phase function of the diffractive optical element is defined by the following equation:
xcfx86(h)=(2xcfx80/xcex)xcexa3Cihi
(where xcex is a reference wavelength, h is a height from the optic axis, and i is a degree)
and where a focal length at the shortest focal length of the overall system is fw,
the following condition is satisfied:
0.005 less than |C2xc3x97fw| less than 0.03
(C2 less than 0);
zooming from the shortest focal length extreme to the longest focal length extreme is effected by moving the first unit along a convex locus on the image surface side and monotonically moving the second and third units toward the original surface, and an object-image distance is constant;
in order from the original surface side, the first unit has two lenses, a (1-1) negative lens and a (1-1) positive lens, the second unit has three lenses, a (2-1) positive lens, a (2-2) positive lens, and a (2-1) negative lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens;
in order from the original surface side, the first unit has two lenses, a (1-1) negative lens and a (1-1) positive lens, the second unit has four lenses, a (2-1) positive lens, a (2-2) positive lens, a (2-1) negative lens, and a (2-2) negative lens, and the third unit has two lenses, a (3-1) positive lens and a (3-1) negative lens; and so on.
An image scanner of the present invention is characterized in that the zoom lens described in either one of the above is used for the image scanner.